No-Red Mahjong

Usage

import mahjax

env = mahjax.make("no_red_mahjong", observe_type="dict")

or load the environment class directly:

from mahjax.no_red_mahjong.env import NoRedMahjong

env = NoRedMahjong(round_mode="half", observe_type="dict")

Description

no_red_mahjong is a fast 4-player riichi mahjong environment without red fives. It is the lightweight environment used by the current offline RL and PPO examples.

Compared with red_mahjong, this environment intentionally omits some rules in order to keep the implementation simpler and faster.

Rules

The environment is a simplified Japanese riichi mahjong variant without red fives.

• No red fives
• No special abortive draws (特殊流局)
• No pao
• No double ron

This environment exists primarily for speed and for simpler RL experiments. It is covered by hand-written tests, and we do not expect major rule-breaking bugs, but some corner cases may still remain. We expect this to improve over time.

For general riichi mahjong rules, see the European Mahjong Association rulebook.

State Representation

no_red_mahjong currently uses the older flat state layout. Mahjong-specific fields live directly on the state object, such as _hand, _river, _melds, _score, and _action_history.

This differs from red_mahjong, which uses a newer nested representation with players and round_state. These two environments therefore do not currently share the same internal state representation, though we may unify them in the future.

Specs

Name	Value
Version	beta
Number of players	4
Number of actions	79
Observation types	dict, 2D
Dict action history shape	(3, 200)
Reward shape	(4,)
Reward semantics	score deltas in hundreds of points

Dict Observation

The current training examples use the dict observation. It is the most stable observation format in this repository right now, but it may still change in future releases.

The returned dictionary contains:

Key	Shape	Meaning
hand	(14,)	Current player's hand as sorted tile types in [0, 33]; unused slots are -1.
last_draw	()	Last drawn tile in [0, 33]; -1 means there is no drawn tile to expose.
action_history	(3, 200)	Action history in the same style as red_mahjong.
shanten_count	()	Current player's shanten number.
furiten	()	Whether the current player is in furiten.
scores	(4,)	Scores ordered from the current player's perspective.
round	()	Round index used by the environment.
honba	()	Honba count.
kyotaku	()	Riichi stick count.
prevalent_wind	()	Current round wind information used by the environment.
seat_wind	()	Current player's seat wind information used by the environment.
dora_indicators	(4,)	Dora indicator tile types in [0, 33]; missing entries are -1.

Action History

action_history is stored as:

• Row 0: acting player index, converted to the current player's relative view
• Row 1: action payload
• Row 2: tsumogiri flag

The semantics match red_mahjong:

• For discards, row 1 stores the actual discarded tile
• For non-discard actions, row 1 stores the raw action id
• Row 2 is 1 for tsumogiri, 0 for a non-tsumogiri discard, and -1 for non-discard actions

For no_red_mahjong, discard tiles are in [0, 33] and raw action ids are in [0, 78].

2D Observation

observe_type="2D" exists, but its design is not yet fixed. You can use it for experiments, but we do not recommend treating it as a stable interface yet.

Action

The action space is:

Range	Meaning
0-33	Discard a tile type
34-67	Closed kan / added kan
68	TSUMOGIRI
69	RIICHI
70	TSUMO
71	RON
72	PON
73	OPEN_KAN
74-76	CHI_L, CHI_M, CHI_R
77	PASS
78	DUMMY

Rewards

Rewards are 4-player score deltas, represented in hundreds of points.

Examples:

• winning a hand gives positive reward to the winner and negative reward to the payer(s)
• exhaustive draw can produce tenpai / noten payments
• illegal actions end the game immediately with the standard illegal-action penalty

Termination

• round_mode="single" terminates after the first round ends.
• round_mode="east" runs East-only progression with round_limit=4.
• round_mode="half" runs East-South progression with round_limit=8.